The present invention relates to the field of high accuracy real-time positioning and more particularly to real-time three-dimensional positioning of a scanning probe microscope with sub-micron scale accuracy.
The invention presented here is in the field of scanning probe microscopy (SPM) and addresses the correction of unwanted deviations of the probe position. FIGS. 1A and 1B illustrate schematically a typical prior art SPM setup. Probe tip 10 sharpened to apex 11 often of atomic scale dimensions is brought into close proximity with sample surface 12, until sensing device 13 detects a desired local interaction between apex 11 and sample surface 12. For scale, the rounded features in probe tip 10 represent typical sizes of individual atoms of the probe. Scanning probe tip 10 across sample surface 12 while keeping the interaction strength constant by means of feedback loop 14a optionally including feedback stop 14b to interrupt the feedback signal, generates a contour map of constant probexe2x80x94sample interaction strength, for example contour lines 15 representing constant interaction scans of probe apex 11 over sample surface 12 in FIG. 1A. Contour map 15 can typically be displayed on monitor screen 18.
In FIG. 1A are shown X, Y, and Z axes 19, which define a coordinate system accepted as conventional in the art. X and Y axes are any two convenient intersecting axes in an XY plane substantially parallel to sample surface 12, whereas Z is a vertical axis intersecting the XY plane that measures height above sample surface 12. The scan pattern of probe 10 over sample surface 12 is conventionally always taken to be in the XY plane, but can be rotated at any angle within the XY plane.
Holding the tip at a fixed location in X, Y and Z with the feedback stop activated is necessary to measure the influence of other parameters on the previously mentioned interactions. Tunneling spectroscopy, which determines the tunneling current as a function of tunneling voltage, is one example.
Examples of localized interactions or processes and the corresponding microscope instruments are: electron tunneling (scanning tunneling microscope, STM), van der Waals and atomic repulsive forces (atomic force microscope, AFM) and capacitive displacement currents (capacitance microscope). In STM and AFM the acquired map or image 15 will reveal detailed surface structure, in some cases with atomic scale resolution. This high resolution mandates mechanically stiff construction and high accuracy probe positioning capabilities, represented by control computer 16 and positioner/scanner 17.
Stability requirements are even more severe for tunneling spectroscopy: decreasing the distance between tip and sample by only 10 pm (one hundred-thousandth of a micrometer) will, under typical conditions, increase the tunneling current by approximately 25 per cent. Similar severe deviations exist for other measurements, which are performed with the feed back stopped. Therefore, mechanically stabilizing the gap between tip and sample is very important.
No prior art method to actively stabilize the distance between tip and sample with a stopped feedback has been described. Hence, scientists and other SPM users who depend on gap stability are forced to work at cryogenic temperatures. Cyrogenic environments require complex and expensive equipment. Furthermore, some environments, e.g. liquids and gases other than helium, are not compatible with ultra-low temperatures. Prior art is limited to tracking of features, corrections in X and Y to improve image acquisition, and post acquisition data processing.
U.S. Pat. No. 5,077,473 (hereinafter the ""473 patent) issued Dec. 31, 1991 to Elings et al., the disclosure of which is hereby incorporated herein by reference, describes prior art techniques to control the XY position of the probe tip of a scanning probe microscope (SPM). The rate of error is determined and is used to obtain a prediction for the needed supplemental signals and then a real time compensation. The inventors cite a technical publication by Pohl et al., xe2x80x9cTrackingxe2x80x9d Tunneling Microscopy, Rev. Sci. Instrum. V59, p. 840 (1988), describing tracking of very small features of a sample, but do not suggest that automated tracking can be used to determine drift rates. Pohl et al. suggests to oscillate the tip of an STM in a circular pattern and apply a lock-in technique to derive the sample slope in X and Y directions from the tunneling current. The X and Y offsets are continuously adjusted to move the tip to the highest (or lowest) point on the sample and thereby lock the tip onto the target extremity. Pohl et al. comments that their tracking method reveals drift and fluctuations. A similar scanning tunneling microscope setup has been used by Aketagawa et al., xe2x80x9cTracking and Stepping Control of the Tip Position of Scanning Tunneling Microscope by Referring to Atomic Points and Arrays on a Regular Crystalline Surface,xe2x80x9d Rev. Sci. Instrum. V70, p. 2053 (1999), to lock the STM tip onto an atom of a graphite sample surface used in their experiment. Like Pohl et al., they limit their discussions to the X and Y coordinates and do not use the obtained drift signals for real time error compensation.
In the prior art, users were not concerned with compensating in the direction normal to the sample surface (Z). They were primarily concerned with imaging applications showing surface details, but not necessarily accurate height information. Most height errors were corrected in the case of imaging by subsequent image processing, e.g., background subtraction, without requiring real time processing. For molecular, atomic or other nanoscale manipulation carried out with an SPM, especially when the Z-feedback has to be disabled during manipulation, deviations in tip-sample distance can lead to complete failure. If for example the tip of an STM is brought closer to the sample surface by only 0.1 nm, the tunneling current will increase by an order of magnitude under typical experimental conditions. The resulting extremely high fields and current densities will often lead to destruction of the tip or of the sample in the area opposing the tip apex, where the atom, molecule, or nanostructure to be manipulated is located. If, on the other hand, the tip is withdrawn, current densities and fields diminish, and the target object may not be manipulated at all. Accordingly, the parameter window for successful manipulation is often narrow, as indicated by the fact that only very robust molecules, for example carbon monoxide, have to date been picked up with the probe of an SPM, and then only at cyrogenic temperatures.
Some measurements carried out with SPMs require that the Z feedback be disabled. Tunneling spectroscopy force-distance curves and measurements of the tunneling current as a function of tip-sample distance are examples. Typically, the relationship between the measured quantity and tip-sample distance is very complex, and post-acquisition data processing cannot be used to correct errors due to deviations of the tip-sample distance.
Accordingly, it is desired in the art to develop a system and method to stabilize and accurately control the position of a SPM probe three dimensionally in real time.
The present invention is directed to a system and method which extend the two-dimensional prior art to provide three-dimensional real time stabilization of the gap between probe tip and sample in a scanning probe microscope (SPM) against drift. The method applies supplemental signals to each of up to three mutually intersecting axes to provide supplemental movement to the probe tip in each axis to offset drift motion relative to a sample. The supplemental signals can be applied simultaneously or sequentially in any combination in the respective axes. In embodiments of the invention, the supplemental signals are determined in response to calculated drift predictions, based in turn on drift measurements inferred from measurement and feedback in response to the gap-dependent strength of an interaction between the probe and the sample. In some embodiments, a waveform modulation is coupled into the drive circuitry for one or more of the axes, and a waveform-synchronous feedback signal is extracted and processed to measure drift. The waveform modulations can be identical or asynchronous and applied sequentially or simultaneously in any combination to the axes. An algorithm performs the process in real time.